CAT Critical Reasoning Practice question with Solution 81

If we are to expand the exploration of our solar system our next manned flight should be to Phobos, one of Mars's moons, rather than to Mars itself. The flight times to each are the same but the Phobos expedition would require less than half the fuel load of a Mars expedition and would, therefore, be much less costly. So, it is clear that Phobos should be our next step in space exploration.

Which one of the following, if true, would most help to explain the difference in fuel requirement?

[A]. More equipment would be required to explore Phobos than to explore Mars.
[B]. Smaller spaceships require less fuel than larger spaceships.
[C]. Information learned during the trip to Phobos can be used during a subsequent trip to Mars.
[D]. The shortest distance between Phobos and Mars is less than half the shortest distance between Earth and Mars.
[E]. Lift-off for the return trip from Phobos requires much less fuel than that from Mars because of Phobos weaker gravitational pull.
Answer: E

The question stem tells us to concentrate not on the conclusion, but on the seemingly unusual relationship posited in the evidence. That is, although the author concludes that Phobos should be next, the testmakers don’t really care about that; they’re simply looking for a reason why, flight times being equal, the Phobos trip requires so much less fuel than does the Mars trip. Well, if the flight times are equal, odds are there’s some other difference between the flights that accounts for the difference in fuel requirements. (E) gives us what we want: If lift-off for the return trip from Phobos would use much less fuel than lift-off from Mars would, then the mystery of the difference in fuel requirements is solved.

(A) and (C) are irrelevant, since neither says anything about the fuel requirements of the two trips.

(B) We have no idea what size ship is needed for a Phobos trip or a Mars trip, so (B) doesn’t help us understand why the two trips of equal flight times have different fuel requirements.

(D) Huh? This confusing mess is an irrelevant comparison. It doesn’t matter in the least how far Phobos is from Mars; the fact that they are relatively close to each other doesn’t help explain the difference in fuel requirements for a trip from Earth to each of these destinations.

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